1. Field of the Invention
2. Related Art
The need for gamut mapping arises in cross-media/cross-device image reproduction. The difference in gamut mapping among different devices/media requires re-mapping of colors such that colors in the source gamut are mapped to within the destination gamut so that they can be reproduced on the destination device/medium. In addition, there are many different ways to achieve this, resulting in different “rendering intents.” For example, ICC (International Color Consortium, www.color.org) profiles support four different rendering intents, called Perceptual, Relative Colorimetric, Saturation and Absolute Colorimetric. The perceptual rendering intent strives at optimizing color reproduction for photographic images. The present invention involves gamut mapping for the perceptual rendering intent.
While the rendering intent sets up a directive for the design of the gamut mapping algorithm, it does not in general define a particular algorithm. Perceptual intent, for example, can be implemented with several possible algorithms. One of the algorithms suggested by CIE TC8-03 division is the SGCK (“Sigmoidal Gaussian Cusp Knee”) algorithm. This algorithm is a combination of the GCUSP algorithm by Morovic (J. Morovic and M. R. Luo: Gamut Mapping Algorithms Based on Psychophysical Experiment, Proceedings of the 5th IS&T/SID Color Imaging Conference, 44-49 (1997)) and the Sigmoidal lightness rescaling algorithm by Braun (G. J. Braun and M. D. Fairchild: Image Lightness Rescaling using Sigmoidal Contrast Enhancement Functions, Journal of Electronic Imaging, Vol. 8, 380-393 (1999). These articles are incorporated herein by reference. The SGCK algorithm is known and in general comprises two steps: a lightness rescaling step, and a chroma compression step.
The SGCK algorithm works well for gamut boundary descriptors (GBD) that have a single convex hull in both the source and the destination. Recently, however, GBDs have been described that have multiple hulls, or “shells”. See U.S. Application Publication No. 2005/0047648, “Color Descriptor Data Structure” by Newman et al., and U.S. Application Publication No. 2005/0047654, “Gamut Mapping Between Multiple Boundary Regions” by Newman et al. These published applications are incorporated herein by reference.